What are viruses characterized as?

Viruses are characterized as tiny infectious agents that cannot replicate on their own and must infect living cells to multiply. They lack the cellular structure found in bacteria and other organisms, making them fundamentally different. Viruses consist of genetic material, either DNA or RNA, surrounded by a protein coat, and sometimes an outer lipid envelope. This unique structure and their dependency on host cells for reproduction define their characteristics.

In contrast to the incorrect options, bacteria are larger and have the capability to replicate independently, often in various environments, including dead cells. Fungi are a separate kingdom of life and are typically composed of complex, multicellular structures, thriving in moist environments, but they are not characterized as viruses. The description of multicellular organisms with no known function does not apply to viruses either, as they serve a distinct purpose in biological systems, despite being simple in structure.

Understanding these definitions highlights the nature of viruses as obligate intracellular parasites, fundamentally showcasing their reliance on living cells for propagation.